SHACL Tutorial: Getting Started

SHACL Tutorial: Getting Started

What is SHACL?

SHACL (Shapes Constraint Language) is a data modeling language that has been developed by a W3C Working Group. As more people work with data coming from a variety of sources, especially for data integration projects, SHACL gives them a way to describe the “shapes” of the data that they’re working with so that applications can take better advantage of that data. In addition to describing which properties go with which classes (for example to drive user input forms), SHACL lets you define constraints on data that, when used by your applications, can make it easier to improve the quality of your data with standardized models instead of procedural code.

This tutorial is written for people who would like to get an overview of SHACL. You will learn how to define some shapes and constraints on those shapes using a few of the properties defined by the SHACL specification. If you do a quick skim through the specification’s table of contents, you will get an idea of the broad vocabulary that SHACL offers to let you describe your data and constraints.

You can use SHACL to define classes together with constraints on their properties. SHACL comes with several built-in types of constraints such as cardinality (minCount/maxCount), value type and allowed values, but it is also possible to define more complex kinds of constraints for almost arbitrary validation conditions. Using these constraints you can communicate the intended format of your data to other people and tools. SHACL validation tools can verify whether your data fulfills the constraints described by your data model, similar to how XML Schema or JSON Schema are being used.

While languages like XML Schema are limited to tree structures, SHACL is based on RDF and supports the validation of graph-based and object-oriented data. Since SHACL is based on RDF, it may also serve as a schema language for JSON via its JSON-LD dialect. The examples in this Tutorial are using the RDF Turtle notation. We also show screenshots of TopBraid Composer, for those who would like to follow along with a graphical editing tool.

If you prefer to experiment with SHACL or follow this tutorial through a simple web interface, please take a look at the SHACL Playground (in JavaScript).

Getting Started

A SHACL file is an RDF graph, which means that it comprises of RDF triples, which each consist of a subject, a predicate and an object. The nodes in a SHACL graph can have a URI, which means that any class, shape or constraint can have a global identifier that can be referenced from other SHACL graphs and shared on the Web as Linked Data.

Let’s start by creating a new SHACL file with a classical data model about Persons and family relationships. This following discussion could also be applied to an existing RDFS or OWL ontology – SHACL can be incrementally applied to existing data models. Assuming you have TopBraid Composer 5.3 installed, create a new project (File > New > Project… named example.org) and then use File > New > SHACL Shapes File as shown:

The Turtle source code so far is merely defining a few namespaces and an owl:Ontology element to capture information about the graph/file itself. It also links to the DASH Data Shapes Library, which extends the SHACL Core Vocabulary, via owl:imports, so that SHACL tools know which library to use:

Back in TopBraid Composer, we can close the Properties panel to gain some screen real estate. Also, switch to the Shapes View on the left hand side. It should now look (roughly) as follows:

Classes, Shapes and Property Constraints

In RDF data models, a Class is a collection of instances with similar properties. Similar to object-oriented systems, SHACL class definitions can define attributes and relationships. Shapes are similar to classes: they also define shared characteristics of a group of nodes and properties that belong together. The main difference however is that shapes are not directly instantiated, but serve as a collector of constraints for validation purposes. Another difference is that shapes can apply to literal nodes, too. Shapes are also useful when we cannot rely on type relationships in the data to determine which constraints apply.

Let’s look at an example. We want to create a class family:Person and specify that each Person must have exactly one last name and one or more first names. In TopBraid, we click Create Shape Class… and fill in the dialog as follows:

Note that this new class has two types. In addition to being a normal rdfs:Class, it is also declared to be a sh:NodeShape. This is a convenient pattern to attach property constraints to a class, although there are also other design patterns to relate classes with the shapes of their instances. When you look at the form of the new class, you can see two types of constraints:

sh:property: Links a shape/class with the associated property declarations, for example the first name of a Person.

sh:sparql: Arbitrary constraints expressed in SPARQL.

In addition to those, a shape may declare constraints that are directly about the target nodes of the shape itself. Examples of this include properties such as sh:closed, sh:and, sh:or and sh:not.

Datatype Property Constraints

In order to state that every instance of family:Person must have exactly one last name, we can click the context menu behind sh:property and select Create Property constraint…, and fill in the dialog as follows:

The resulting source code now contains a class definition with one property constraint. Note that the property constraint is represented by an anonymous “blank” node, as shown by the […] angular brackets. If you prefer to use URIs for these nodes instead, use Convert to URI node from the context menu behind the property constraint.

The shape above has introduced the use of some properties defined by the SHACL specification:

sh:path: Points at the URI of the property that is being restricted. Alternative, it may point at a path expression, which would allow you to constrain values that may be several “hops” away from the starting point.

sh:minCount and sh:maxCount: Can specify the minimum and maximum number of allowed values for the property, aka the cardinality. If left unspecified, the cardinality is zero-to-many.

sh:datatype: Specifies that all values must be literals with the given type. Common values are xsd:string, xsd:boolean, xsd:integer, xsd:decimal, xsd:date and xsd:dateTime. Use rdf:langString for strings with a language tag, and rdf:HTML for strings including HTML markup.

In addition, property constraints may define human-readable labels via sh:name and descriptions of the property via sh:description, both potentially in multiple languages.

We can repeat the same mechanism to declare family:firstName – the only difference is that we define the cardinality to be One or more [1..*]. Once we have done this, the resulting class definition looks as follows in TopBraid:

In the screenshot above, the details of the lastName property are expanded. This can be achieved by moving the mouse over the property icon, and clicking the + button.

Let’s now use this class and shape definition for some validation task. We can use TopBraid to create an instance of the class family:Person, e.g. on the Instances panel. An example instance could look like the following:

The source code of the Person instance above is the following:

family:JohnDoe
rdf:type family:Person ;
family:lastName "Doe" ;
.

This instance violates the sh:minCount constraint at family:firstName. In TopBraid, you can activate the “exclamation mark” Display constraint violation warnings button in the tool bar to highlight incorrect or missing values on the form. You can also display a clickable list of all constraint violations in the corresponding SHACL Validation panel at the bottom of the screenshot above. Adding a first name will make the errors disappear.

Relationship Property Constraints

SHACL property constraints may also define relationships between resources. For example, a Person may have a mother and a father, which are again instances of Person. Let’s create the family:mother relationship and leave the father as an exercise. In TopBraid Composer, navigate back to the Person class and pick Create Property constraint… in the drop down behind sh:property, then fill in the dialog as follows:

The resulting source code now has an additional constraint on the mother property:

sh:nodeKind: Can be used to specify that all values of a property must be Literals, IRIs or Blank nodes. Here, we have selected sh:IRI to make sure that all values are resources with an IRI (that is, a URI that allows an internationalized character set), and not an anonymous node that cannot be linked to from other graphs.

sh:class: Specifies the type (rdf:type) of all values. In other words, all values of mother must be instances of family:Person, or a subclass thereof.

In order to make this example more interesting, let’s create another class family:Gender with two instances family:male and family:female.

Next, create a new property family:gender that points from the Person class to the Gender class:

Now we are ready to explore pure Shapes that are not (also) classes. Let’s assume we want to specify that all values of family:mother must be female. To do this, we create a new sh:NodeShape called family:FemaleShape with a constraint on the family:gender property. In TopBraid Composer’s Shapes view, click on Create Shape…, leading to the following dialog:

This takes us to the form for family:FemaleShape, where we can add a property constraint on family:gender. This time we select Add empty row in the context menu behind sh:property on the form. Then we enter family:gender under sh:path, and select family:female using Add existing value… next to sh:hasValue, leading to the following definition:

The source code of the complete definition of this shape looks as follows:

The sh:hasValue system property represents a constraint that one of the values of the given predicate must be family:female. (As a side effect this also implies that at least one value must exist).

We can now go back to the Person class and add a sh:node constraint to the family:mother property. To do that in TopBraid Composer, open the “nested” form for the property constraint using a right-click so that all relevant properties including sh:node appear. (Alternatively, edit the Source Code tab directly). Finally, add family:FemaleShape to the values of sh:node, leading to the following situation:

The meaning of sh:node is that all values of the property must fulfill the constraints defined by the given sh:NodeShape. This is very different from sh:class in that it actually “walks” into the values of family:mother to (recursively) check that they all have the female gender. Such shapes can become quite complex and may include further nested shapes. SHACL also has a syntax to express logical combinations such as AND, OR and NOT for complex scenarios.

Anyway, we can now verify that our constraint actually works by creating a simple constraint violation. We can make John Doe his own mother. This should trigger a violation because the mother must be female:

Further Topics

There would be much more to say about SHACL in this tutorial. Here is a list of some notable features for future editions and further reading:

Targets (sh:target) are a general mechanism for SHACL to select the nodes to which a given Shape applies to. A common pattern is to associate a Shape with a class (e.g. via sh:targetClass) but this mechanism can also be used to associate a shape with all subjects that have a given property, thus defining “global” property constraints.

User-defined constraint components are a macro facility in SHACL, allowing advanced users to prepare new high-level modeling elements to describe constraints. The SHACL Core constraints such as sh:minCount are all defined using its own extension mechanism.

Functions are another macro facility to define new SPARQL functions that encapsulate another SPARQL query similar to stored procedures. This can lead to dramatically more maintainable queries based on reusable building blocks.